1. Field of the Invention
The present invention relates to an improved method for producing microcapsules and, more particularly, to a method for producing microcapsules in which the wall is formed from a formaldehyde condensation product. While the method of the present invention is useful in producing microcapsules generally, microcapsules prepared in accordance with the present invention are particularly useful in photosensitive imaging materials of the type which employ a layer of microcapsules containing a radiation sensitive composition.
2. Brief Description of the Prior Art
Imaging materials employing photosensitive microcapsules ar subject of commonly assigned U.S. Pat. Nos. 4,399,209 and 4,440,836.
In the aforesaid commonly assigned U.S. patents, images are formed by image-wise exposing a layer of photosensitive capsules to actinic radiation and rupturing the capsules typically by passing the imaging sheet containing the capsules through a pressure nip. The radiation sensitive composition contains a photohardenable or photosoftenable material which undergoes a change in viscosity or modulus upon exposure. For example, in the most typical embodiments, the radiation sensitive composition contains a polyethylenically unsaturated monomer which polymerizes upon exposure, thereby causing the internal phase of the capsules to become harder. Due to the difference in the hardness of the capsules in the exposed versus the unexposed areas, only certain capsules rupture and release their contents. If the internal phase contains a dye precursor, the precursor is image-wise released, and a color image is formed upon its transfer to a developer layer. In previously disclosed embodiments, the developer layer may be present on the same support as the layer of capsules or a separate support.
As a microcapsule wall material, amine-formaldehyde polymers, and more particularly, melamine-formaldehyde polymers, have been used to encapsulate the photosensitive internal phase. For example, U.S. Pat. No. 4,608,330 discloses the encapsulation of photosensitive materials of the types described above in melamine-formaldehyde capsules. The process for preparing the microcapsules typically involves forming an emulsion of an oily internal phase composition in an aqueous phase, and enwrapping particles of the internal phase in a formaldehyde condensation product produced by in situ condensation of melamine and formaldehyde. This patent also discloses that a polyisocyanate may be added to the oily internal phase composition. The isocyanate forms a polymer pre-wall around the internal phase upon introduction into the aqueous phase.
Similarly, U.S. Pat. No. 4,353,809 discloses microcapsules with melamine-formaldehyde or urea-formaldehyde capsule shell walls. The reference also teaches the addition of a polyvalent isocyanate, a polyvalent isothiocyanate or a prepolymer thereof to the internal phase prior to addition to an emulsion to form a pre-wall material around the internal phase. The presence of the isocyanate, isothiocyanate, or prepolymer functions to stabilize the emulsion and enable the production of small diameter microcapsules.
Although the above microencapsulation processes have proven to be of great importance, they still suffer from one or more drawbacks.
For example, the polycondensation of the isocyanate groups in forming the polyurea pre-wall takes a relatively long time, and any reaction which occurs is confined to the oil-water interface. It is believed that as a thin shell of a pre-wall is formed, pre-wall formation stops. It is believed that it would be desirable for this reaction to continue to form a thicker pre-wall, particularly when the microcapsules are used in photosensitive materials.
When the internal phase of the microcapsules contains a photosensitive composition, other problems must be considered. Free radial initiated reactions are generally relied upon to change the physical characteristics of the microcapsules and thereby form the image. These reactions are inhibited by oxygen. The microcapsule walls function as a barrier material to prevent oxygen from infiltrating the photosensitive composition. Upon exposing the material to actinic radiation, free radicals consume the oxygen present in the composition and reactions proceed. If oxygen readily re-infiltrates the composition, the photographic speed of the material is very poor.
It has been found that the microcapsule walls should be water resistant. It is believed water can plasticize the wall material and reduce the wall's barrier properties. If the barrier properties are reduced, the amount of oxygen diffusion will increase and, in turn, the photographic efficiency of the internal phase will significantly decrease. Previous microcapsules, although generally effective, tend to lose their photographic speed when they are stored under high temperature, high humidity conditions.
Accordingly, there exists a need in the art for microcapsules having improved high temperature/high humidity characteristics (e.g., 30.degree. C./80% RH).